Electrolytic cladding of zirconium on uranium



2,905,599 I ELECTROLYTIC CLADDING F zIRcoNiUM oN URANIUM Jerome J. Wick, Cleveland, Ohio, assignor, by m'esne assignments, to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application February 15, 1956 Serial No. 566,057

4 Claims. .(Cl. 204-1.5)

This invention relates to the electrolytic formation of adherent zirconium coatings on uranium metal specimens. The use of uranium metal in nuclear reactor technology has led to a need for protective coatings on the metal in order to minimize or prevent reaction of the uranium with cooling media or other substances which may pass into contact therewith while it is in use in reactors. In order to prevent a reaction it is necessary that the protective coating be continuous as damage will result if reaction can proceed through an opening as small as a pinhole.

It is accordingly one of the objects of the subject invention to provide a method of forming a relatively thin adherent continuous coating of zirconium metal on uranium. Another object is to provide a method of forming an adherent continuous ductile cladding of zirconium metal on uranium capable of protecting the uranium metal from chemical attack by corrosive agents. A further object is to provide a uranium element having a continuous ductile zirconium metal coating and a zirconium-uranium diffusion layer bonded to the surface thereof. Still another object is to provide a method of forming a relatively thick adherent continuous coating of zirconium metal on uranium. Other objects will be in part apparent and in part pointed out hereinafter.

In one of its broader aspects the objects of the present invention are accomplished by rendering the surface of the uranium article to be coated substantially oxide-free and smooth, maintaining the article in an oxygen-free atmosphere, immersing it into a molten electrolytic bath consisting essentially of NaCl, K ZrF KF, and ZrO and before the article reaches temperature equilibriumv with the bath, applying an electrolyzing current of approximately 60 amperes per square decimeter thereto through said bath at a voltage of approximately 3 volts until a layer of zirconium metal is formed on the surface of the article.

It has been found that in order to form zirconium metal coatings having a thickness greater than 5 mils on uranium elements, certain critical plating conditions must be observed. The ratio of the bath constituents, the temperature and time of the plating, the voltage and amperage employed, the time at which they are employed, and the preparation of the metal uranium specimen to be coated have all been found to critically affect the results obtained and failure to observe the conditions specified results in formation of cladding of smaller dimensions or no coating at all.

It is important to observe a number of precautions in carrying out the plating process in addition to those relating to the time, temperature and current, as pointed out above. In preparing the specimen it is critically important to finish the surface of the metal so that there are no cracks or seams larger than .5 mil. Any cracks will ruin the clad no matter how small. It is possible to render the surface sufficiently smooth for plating a continuous coating by surface grinding the specimen if it is abaiii Patented Sept. 22, 1959 2 of regular shape or electropolishing the surface if it is of irregular shape.

It has been found necessary in addition, if an adherent or bonded coating is to be produced, to carry out the plating operation on a clean metal surface. For this reason it is necessary to remove any scale or oxide from the metal surface by acid leaching or other similar metal cleaning procedure after the surface is rendered smooth. Dipping the speciment in dilute nitric acid or hydrochloric acid has been found satisfactory for this purpose.

After the surface has been rendered scale-free and oxide-free it is necessary to avoid exposure of the surfaceto atmosphericcontamination because of the strong tendency of this metal to form an oxide surface coating on exposure to air. The surface may be maintained in an electro-platable condition by keeping the speciment in an inert gas atmosphere such as an argon atmosphere until it is immersed in the fused salt bath. The specimen is preferably immersed in the salt with the electrolyzing current off to avoid any arcing with the crucible walls or salt surface as the pits formed by the arc could make its surface unfit for the formation of a continuous corrosion proof electroclad.

I have found that it is necessary to immerse the metal into the fused salt without preheating in order to form a bright continuous clad. The cold immersion causes a salt cake to form on the specimen surface temporarily, i.e.,. until the specimen reaches bath temperature and the salt melts from the surface. The presence of the salt cake assists in the initial application of current to the specimen. A finer degree of control over the initial electrocladding currents can be achieved by applying the voltage after the specimen is immersed in the salt but before the cake formed around the specimen completely melts and. the specimenreaches temperature equilibrium with the bath. The control of these currents is important as the success of the operation depends on the successful initiation of a continuous bright metal clad rather than a loosely adherent particulate material. I have found that when the sample is allowed to reach bath temperature be fore starting electrolysis no clad is produced whereas in the same bath a good clad can be produced, using the same operating conditions, by initiating electrolysis before the sample reaches bath temperature. My most consistent and best results have been obtained by passing the electrolyzing current through the system immediately after the specimen is immersed in the bath and this is there fore my preferred procedure.

It is important in addition to provide adequate electrical contact and support for the specimen in the bath. Con:- ventional clamps as well as basket and spring clamps can'- not be employed conveniently because of the high ternperature of the bath and the pitting and arcing at the contacting surfaces which rendered the clad discontinuous. at these points. It is preferable to employ a clamp con sisting of a metal or combination of metals which have a coefiicient of expansion corresponding to that of uranium. A two bolt conventional yoke or vice-type clamp composed of steel and provided with a number of molybdenum and copper shims in such ratio as to have a combined coefficient of expansion corresponding to that of uranium is satisfactory for this purpose. It is preferable to maintain electrical contact between the clamp and clad surface of the specimen through an inert metal such as molybdenum in order to prevent any reaction with the zirconium clad which might lead to its deterioration.

It is satisfactory for formation of a protective cladding amenable to hot working to employ reagents of normal reagent grade although it is desirable that they be free of any moisture, nitrogen or oxygen as this tends tointerfere with the formation of continuous bright coatings. The formation of cold-workable coatings, however, requires special preparation or high purity of the reagents employed. Insoluble impurities in the K ZrF charge should preferably be kept below 0.01% to produce-a high-purity and low-hardness clad consistently. One procedure which has been found satisfactory in purifying the materials is to subject them to a preliminary, low-potential electrolysis prior to attempting .to coat the metal specimen with bright zirconium. A ptentialof about 2 volts is satisfactory for this purpose. Alternatively the materials may-be individually selected or treated, as, for example, by recrystallization, to insure proper purity and dryness. The advantages possible of attainment in carrying out the method of the subject invention may be brought out with reference to the following examples of formation of relatively thick and relatively thin coatings on a uranium specimen.

Example I An electrolytic bath composed of fused salts is prepared by mixing together 4 pounds of NaCl, 2 pounds of K ZrF 1 pound of KF and 25 grams of ZrO All of the reagents used were of high purity, dry and free from oxygen and nitrogen. This mixture is heated to a temperature of about 800 in a graphite crucible, the crucible itself serving as anode for the electroplating. The uranium specimen to be coated is rendered smooth by surface grinding or burnishing with a. wire wheel. It is then degreased in acetone, etched in a l to 3% solution of HCl to remove surface oxidesor slags, rinsed in ,hot water and dried in acetone. After drying in acetone it is removed from air as quickly as possible to protect it from the formation of a U0 surface layer. The specimen is then clamped and immersed into the bath, while cold and an atmosphere of argon is passed over the bath. The voltage is applied immediately after the specimen is fully immersed. A current density of approximately 60 amperes per squaredecimeter and a voltage of approximately 2.7 volts are applied for-approximately 15 minutes. The plated specimen is raised from the salt bath and allowed to cool in an atmosphere of argon. The salt-metal mixture adhering to the specimen surfaceis chipped away and the specimen is soaked in hot WatertO remove remaining surface salts. An electroclad having a thickness of about 7 mils was formed during this.l minute electroplating period. In. order to plate the portion of the sample which was held by the clamp the sample is removed, cleaned again as described above and clamped on the clad end. Care must be taken when bufling the partially clad bar not to damage the comparatively soft surfaces of the zirconium clad. Theelectroplating process is then repeated.

Metallurgical examinations of the clad surfaces clearly show the continuous natureof the zirconium layer and the ditfusion bond formed between the layer and uranium metal. Diffusion layers of l to 3 mils in thickness can be formed by this procedure and are found to consist of a uranium-zirconium composition.-

Electroplated uranium bars prepared in this manner have been subjected to corrosion tests. These tests consist of subjecting the clad uranium bar to dilute solutions of HNO l to 3 percent, for times ranging from' l to 36 hours. No signs of' corrosionwere detected.

' X-ray diifractometer patterns obtained on these zirconium clad uranium specimens established that the zirconium is hexagonal alpha zirconium and that the layer contains no uranium. By repeating this procedure,,zir conium metal layers having thicknesses up to 40 mils may be produced according to this preferred procedure. In repeating the electrocl'adding any change in the composition of the bath should be corrected 'tomaintain the composition in the approximate ratioof 72:36: 18:1 for NaCl, K ZrF KF and ZrO respectively.

a It is possible to clad uranium with a tightly bonded zirconium clad and diffusion layer by variation of the cell conditions and composition. However this clad is the order of only one tenth the thickness of the clad formed according to the preferred mode asset out in Example I.

Example II A high purity bath containing 25 to K ZrF and the remainder NaCl is heated to a temperature of between 780 to 830 C. in a graphite crucible. The sample to be coated is prepared as described above to present a smooth clean surface to the salt bath; The sample is plated essentially as described in the example above for about five minutes at a voltageof from 3 to 3.5 volts and at an amperage of -60 to 300 amps./dm. One such cladding which was produced by immersing six times and electroplating for six minutes with each immersion for a total of 36 minutes developed a clad having an average thickness of 1.1 mils. -A difiusionzone of 0.8 mils thickness was also formed as a result of these runs. This diffusion zone had a Yickers hardness number; of 1 245, intermediate between that of uranium ;(215) and zirconi- 11m.(3

-. vBoth the zirconium metal and diffusion zone layers may be increased in thickness by ipcreasing the time of plating. In order to retain the soft ductile property of zirconium metal platedfrom high purity-baths according to the process of the present invention it is preferable tdemploy a series of short runs ratherthan a single long .run. A single run lasting 15 minutes at substantially the same conditions as those in the Example II above, produced a zirconium layer and diffusion zone having Vickershardness values approximately 100. points-higher than the hardness value of the layers produced according to theprocedure of that example, although its thickness was only half of that of the layer produced according .to the without cracking or exfoliation.

example. v .7 Tests of the clads formed according to the present invention showed the layer to be adherent, ductile, and capable of withstanding considerable cold deformation The cyclic heating and cooling incident to the repeated immersion and electroplating carried, out in the series of runs noted above caused no cracking or exfoliation of the formed zirconium layer or diffusion ,layer.,

From the numerous tests conducted 60 amperes per square decimeter appears. to be the optimum current density for forming zirconium clads having the desired properties on uranium metal. However current densities ranging from 20 to amperes per square decimeter are useful for forming tight clads. The lowest voltage which can beused is about 2.5 volts sinceno. electroplating occurs below that value. Voltages above 4.5 should be avoided because of the tendency of polarization to interfere with the cladding above this value. This voltage is of course limited by thesize of the bar used (4" x 1 x A3") and would vary with the size of thebar. Since many embodiments might be made of the present invention and since many changes mightbe made in-the embodiment described, it is to be understood that the foregoing description is to be interpreted as illustrative only and not in a limiting sense. 1

I claim: I

l. The method of cladding uranium with a continuous coating. of a bright zirconium metal which comprises'rendering the surface of the uranium article to be coated substantially oxide-free and smooth, protecting said article from oxidation, immersing the article into a molten elect-rolytic .bath consisting essentially of NaGl, K ZrF KF and ZrO in the weight ratio of 72:36: 18:1, applying an :electrolyzing ,currentof approxim-ately 6O amperes per square decimeter to said article ata voltage of approximately 3 volts before said article reaches temperature equilibrium withsaid; bath until a layer of zirconium metal of desired thickness is formed thereon.

2. Themethod of cladding uranium with a relatively thick continuous coating of bright zirconiumrnetal which comprises rendering the surface of the uranium article to be coated substantially oxide-free and smooth, protecting said article from oxidation, preparing a molten bath consisting essentially of NaCl, K ZrF KF and ZrO in an approximate ratio of 72:36:18z1, subjecting said bath to a preliminary electrolysis at a voltage of 7 volts and a current of 100 amperes :for approximately 2.5 minutes to purify the bath, immersing said article without preheating, applying an electrolyzing current of approximately 60 amperes per square decimeter at a voltage of approximately 3 volts immediately after said article is immersed and until a layer of zirconium metal of desired thickness is formed thereon.

3. The method of cladding uranium with a relatively thick continuous coating of bright zirconium metal which comprises rendering the surface of the uranium article to be coated substantially oxide-free and smooth, protecting said article from oxidation, immersing the article into a molten electrolytic bath maintained at a temperature of about 800 C. and consisting essentially of NaCl, K ZrF KF and ZrO in the weight ratio of 72:36:l8:l, applying an electrolyzing current of approximately -60 amperes per square decimeter at a voltage of approximately 2.7 volts through said bath immediately after said article is immersed and for about 10 minutes thereafter, removing said article from the bath, cleaning the surface thereof to expose the bright zirconium clad and repeating said electrolyzing process until a zirconium clad of desired thickness is formed.

4. The method of cladding uranium with a relatively thick continuous coating of bright ductile zirconium metal which comprises rendering the surface of the uranium article to be coated substantially oxide-free and smooth, forming an electrolytic bath consisting essentially of high purity NaCI, K ZrF KF and ZrO in the weight ratio of 72:36:18:1, heating said bath to a temperature of 800 C., immersing said article into said bath, and applying electrolyzing current of approximately amperes per square decimeter at a voltage of approximately 3 volts through said bath immediately after said article is immersed therein and for about 15 minutes thereafter.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES US. Atomic Energy Commission, AECD3788, by S. H. Bush et al., Nov. 18, 1953, pp. 30-32.

Journal of Electrochemical Society, vol. 101 (1954), pp. 63-69, 77.

The Sylvania Technologist, January 1956, vol. IX, No.

1, January 1956, pp. 2-6. 

1. THE METHOD OF CLADDING URANIUM WITH A CONTINUOUS COATING OF A BRIGHT ZIRCONIUM METAL WHICH COMPRISES RENDERING THE SURFACE OF THE URANIUM ARTICLE TO BE COATED SUBSTANTIALLY OXIDE-FREE AND SMOOTH, PROTECTING SAID ARTICLE FROM OXIDATION,IMMERSING THE ARTICLE INTO A MOLTEN ELECTROLYTIC BATH CONSISTING ESSENTIALLY OF NACL, K2ZRF6. KF AND ZRO2 IN THE WEIGHT RATIO OF 72:36:18:1, APPLYING AN ELECTROLYZING CURRENT OF APPROXIMATELY 60 AMPERES PER SQUARE DECIMETER TO SAID ARTICLE AT A VOLTAGE OF APPROXIMATELY 3 VOLTS BEFORE SAID ARTICLE REACHES TEMPERATURE EQUILIBRIUM WITH SAID BATH UNTIL A LAYER OF ZIRCONIUM METAL OF DESIRED THICKNESS IS FORMED THEREON. 